1. Field of the Invention
The field of art to which the claimed invention pertains is solid bed adsorptive separation processes. More specifically, the invention relates to a process and particular flow scheme for separating C.sub.8 aromatic isomers from a mixture thereof.
2. Description of the Prior Art
It is well known in the separation art that certain crystalline aluminosilicates can be used to separate one hydrocarbon type from another hydrocarbon type. The separation of normal paraffins from branched chained paraffins for example can be accomplished by using a type A zeolite which has pore openings from 3 to 5 Angstroms. Such a separation process is disclosed in U.S. Pat. Nos. 2,985,589 and 3,201,491. These adsorbents allow a separation based on the physical size difference in the molecules by allowing the smaller or normal hydrocarbons to be passed into the cavities within the zeolite adsorbent, while excluding the larger or branched chain molecules. U.S. Pat. Nos. 3,265,750 and 3,510,423 for example disclose processes in which larger pore diameter zeolites such as the type X or type Y structured zeolites can be used to separate olefinic hydrocarbons from non-olefinic hydrocarbons.
In addition to being used in processes for separating hydrocarbon types, adsorbents comprising type X or Y zeolites have also been employed in processes to separate individual hydrocarbon isomers. In the processes described in U.S. Pat. Nos. 3,558,730; 3,558,732; 3,626,020 and 3,686,342 for example particular zeolitic adsorbents are used to separate desired xylene isomers; in U.S. Pat. No. 3,114,782 they are used to separate alkyl-trisubstituted benzenes; in U.S. Pat. No. 3,864,416 they are used to separate tetraalkyl substituted aromatic hydrocarbon isomers and in U.S. Pat. No. 2,668,267 they are used to separate particular alkyl substituted naphthalenes.
The principal of the simulated moving bed to continuously separate the components of the fluid mixture by contact with a solid adsorbent is as set forth in D. B. Broughton U.S. Pat. No. 2,985,589, hereby incorporated herein by reference thereto. The simulated moving bed functions by periodically advancing through the column of adsorbent the various points of introducing and withdrawing the liquid streams.
A common problem with the simulated moving bed flow scheme is that the raffinate itself contains components the separation of which is desired, but which cannot be separated by conventional means, such as fractionation. In such cases, therefore, it has been the practice to provide a second stage simulated moving bed to achieve the complete separation. I have discovered a method of operating a simulated moving bed column which avoids the aforementioned problem that would otherwise occur with certain three component feedstocks.
The process is particularly useful in separating ethylbenzene and ortho, meta, and para-xylene from a mixture in which they occur.